Hot melt adhesives are applied to a substrate while in its molten state and cooled to harden the adhesive layer. Such adhesives are widely used for various commercial and industrial applications such as product assembly and packaging, and have been widely used in the non-woven industry to make baby diapers and adult incontinence products. In these applications, adhesive is applied to at least one substrate such as, for example, a packaging substrate (e.g., cardboard), a non-woven substrate (e.g., polyolefin), or an elastic substrate (e.g., spandex) for binding the substrate to a second similar or different.
A pressure sensitive adhesive is permanently tacky at room temperature and can adhere to a substrate with very light pressure. In other words, it has an infinite open time to enable bonding to the adherent. A hot melt PSA is one type of PSAs that is manufactured and processed at high temperatures in its molten state, and exhibits PSA characteristics after being cooled down and solidified into solids. Styrenic block copolymers such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) have been used commercially as base polymers for hot melt PSAs. In a hot melt PSA formulation, base polymer provides cohesive strength and elasticity. The use of high molecular weight polymer or high polymer content usually promotes cohesive strength and adhesion, however, it also results in significant melt viscosity increase. A hot melt adhesive with high viscosity may require very high processing temperature, at which polymers are susceptible to degradation, charring, gelling and loss of adhesion. High processing temperature is also not preferred for safety concern and cost concern.
While hot melt adhesives are conventionally used a number of industries, there is a need for a high cohesive strength hot melt pressure sensitive adhesive with low melt viscosity that can be used to bond various types of substrates for certain end use applications. The current invention addresses this need.